The effect of calcium on the antioxidant enzymes from salt-treated loquat and anger plants

Salt treatment (50 mM NaCl) reduced plant growth of loquat (Eribotria japonica Lindl.) (by up to 40%) but not that of anger (Cydonia oblonga Mill.). Salt stress induced a strong leaf Na⁺ accumulation in both species. However, the observed increase in leaf Cl^- level was higher in loquat (13-fold) than in anger plants (3.8-fold). Addition of Ca²⁺ (25 mM) significantly reduced Na⁺ and Cl^- concentrations in both salt-treated species. In anger leaves, calcium addition to the nutrient media did not change the leaf calcium contents in salt-treated or untreated plants, this value being lower in salt-treated plants. However, in loquat plants, an increase in leaf Ca²⁺ was observed after the calcium addition. Surprisingly, an increase in Ca²⁺ concentration was also observed in salt-treated loquat plants. In general, anger plants had higher constitutive antioxidant enzyme levels in both control and salt-treated plants. Salt stress did not change antioxidant enzyme levels in loquat plants. A similar effect was observed in anger plants, but in this case a 2-fold induction of monodehydroascorbate reductase (MDHAR) activity was observed.In both species, salinity produced an oxidative stress, indicated by an increase in lipid peroxidation, this value being much higher in loquat (83%) than in anger (40%) plants. In salt-treated plants, Ca²⁺ addition provided some protection to the membranes, because the increases observed in thiobarbituric-acid-reactive substances (TBARS) were not significant.In contrast, in control plants Ca²⁺ treatments increased glutathione reductase (GR) and decreased catalase activity in anger, but increased MDHAR, dehydroascorbate reductase (DHAR), GR and superoxide dismutase (SOD) in loquat plants. In salt-treated plants, Ca²⁺ additions decreased catalase (CAT) and ascorbate peroxidase (APX) for anger and raised DHAR, GR and SOD for loquat. However, the mechanism by which Ca²⁺ regulates antioxidant enzymes remains to be determined.These results suggest that anger plants have a higher capacity to scavenge AOS, both under saline and non-saline conditions. Accordingly, and related to the smaller Cl^- increase observed, anger plants are more salt-tolerant, at least partly owing to the higher antioxidant enzyme levels observed.